Flexure stage alignment apparatus

ABSTRACT

An apparatus for micropositioning an X-ray lithography mask has but a single stage plate for supporting the mask, three piezoelectric transducers for moving the stage plate in the X-Y plane, and three flexure assemblies located equiangularly around the stage plate for supporting the stage plate and for moving the stage plate in the Z-axis. The flexure assemblies each include a single piezoelectric transducer and various flex strips to allow relative motion of the stage plate to occur smoothly in all six of the possible degrees of freedom.

BACKGROUND AND SUMMARY OF THE INVENTION

In the manufacture of semiconductor devices having submicron linewidths, X-ray lithography is often used to replicate a desired patternfrom a mask to a semiconductor wafer. Since different masks are used indifferent processing steps of the wafer, it is of critical importancethat the alignment of each mask relative to the wafer be held to a verytight tolerance.

It is known in the prior art to use a combination of a piezoelectricelement and a pivoting arm in a positioning device having a singledegree of freedom as is shown in an article by Scire and Teague at pages1735-40 of The Review Of Scientific Instruments, 49(12), December, 1978.In order to provide all six degrees of freedom (translation along, androtation about, each of the X, Y and Z axes), alignment apparatusconstructed according to the prior art have utilized series stackedstages in which each stage provides one or several independent degreesof freedom. Exemplary of such a stacked stage alignment apparatus is afive stage apparatus disclosed by Yamazaki et al at pages 987-991 of TheJournal Of Vacuum Science Technology, 15(3), May/June 1978. Since thelength of the structural loop between the mask and the wafer increasesas the number of stages increases, the prior art stacked stage apparatusbecomes more and more susceptible to external vibration as the number ofstages increases.

McCoy et al, in U.S. patent application Ser. No. 469,154, now abandoned,disclose a single stage alignment apparatus capable of providing maskmovement in only three degrees of freedom (translation along the X and Yaxes and rotation about the Z axis). The McCoy, et al apparatus, shownin FIG. 1, uses four piezoelectric elements to support and move themask.

In accordance with the illustrated preferred embodiment of the presentinvention, a single stage alignment apparatus provides six degrees offreedom of an X-ray mask relative to a semiconductor wafer. Since only asingle stage is used the apparatus is smaller and lighter than a stackedstage apparatus with the result that the stiffness of the apparatus isincreased and position control of the mask is improved. The apparatuscomprises three flexure assemblies spaced equiangularly around a stageplate which supports the X-ray mask. Each flexure assembly includes aZ-axis piezoelectric transducer and horizontal and vertical flexureswhich allow the stage plate to move smoothly. Three piezoelectictransducers provide motion of the stage plate in the X-Y plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art alignment apparatus which uses fourpiezoelectric elements to provide three degrees of freedom for movementof a stage plate.

FIG. 2 shows six vectors which may provide six degrees of freedom of astage plate.

FIG. 3 shows a top view of an alignment apparatus which is constructedin accordance with the preferred embodiment of the present invention.

FIG. 4 shows a perspective view of a flexure assembly which is used inthe apparatus shown in FIG. 3.

FIG. 5 shows a side view of the flexure assembly shown in FIG. 4.

FIG. 6 shows a side view of an actuator assembly which may be used inthe apparatus shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a top view of a prior art alignment apparatus which isdisclosed in the McCoy et al U.S. patent application discussed above.

FIG. 2 shows an X-ray mask 1 which is held by a frame 3. Also shown aresix displacement vectors which may be combined to provide the sixdegrees of freedom of mask 1 wherein: X alone provides translation alongthe X-axis; Y₁ and Y₂ acting together provide translation along theY-axis; the difference of Y₁ and Y₂ provides rotation about the Z-axis;and combinations of the three Z vectors provide translation along theZ-axis and rotation about the X- and Y-axes.

FIG. 3 shows an alignment apparatus which is constructed in accordancewith the preferred embodiment of the present invention. Mask 1 and frame3 are attached to a single, rigid stage plate 5 which is positionable inthe six degrees of freedom. Plate 5 may be positioned in the X-Y planeby the use of well known piezoelectric transducers (PZTs) 31, 33, 35which are operative for producing displacement in response to an appliedelectrical potential. For the apparatus shown in FIG. 3, PZTs 31, 33, 35typically provide a displacement of 40 microns for an applied potentialof 1000 volts although the relationship of displacement to voltage isnot necessarily linear and hysteresis effects are common. PZTs 31, 33,35 are connected to stage plate 5 by elastic elements 51, 53, 55, whichare stiff along a longitudinal axis and flexible in all directionswithin a plane normal thereto. Fixed blocks 37, 39, 41 anchor PZTs 31,33, 35.

FIG. 6 shows a side view of an actuator assembly which may be used toprovide PZT displacement amplification. PZT 31 is attached to a fixedpivot assembly 91 and to an arm 99. As PZT 31 is displaced, arm 99pivots about the attachment point of a stiff spring 93 causing a rigidbar 95 to displace stage plate 5. Elastic elements 97 are stiff in alongitudinal direction and flexible in all directions within a planenormal thereto.

Three flexure assemblies 11, 13, 15 are located equiangularly aroundstage plate 5, as shown in FIG. 3. Flexure assemblies 11, 13, 15 allowdisplacement of stage plate 5 in all of the six degrees of freedom withthe use of but a single stage plate 5. Flexure assemblies 11, 13, 15 areconnected to stage plate 5 by flexure strips 17, 19, 21.

FIGS. 4 and 5 show more detailed views of flexure assembly 11 which isidentical to flexure assemblies 13 and 15. A pedestal 61 is connected byfour crossing flexure strips 71 and 73 to an arm 63. Each of flexurestrips 71 and 73 may comprise, e.g., hardened beryllium copper having alength of 0.2 inches, a thickness of 0.045 inches and a width of 0.25inch. The cross points of flexure strips 71 and 73 define an axis aboutwhich arm 63 may be smoothly rotated by a vertical PZT 69. Due to thelocation of vertical PZT 69, a 40 micron vertical displacement (causedby the application of a 1000 volt potential to PZT 69) causes a 160micron vertical displacement of vertical flexure strips 65 at the end ofarm 63.

Rigid bar 67 is attached between vertical flexure strips 65 and a singleflexure strip 17 is attached from bar 67 to stage plate 5. Flexurestrips 65 and 17, which may be fabricated from a spring material, e.g.,hardened beryllium copper, allow relative motion of the stage plate 5 tooccur smoothly and repeatably. Thus, movement of mask 1 in any of thesix degrees of freedom is achieved without jitter or backlash and withthe use of but a single stage plate 5.

The apparatus shown in FIG. 3 may be most efficiently used for aligningan X-ray mask and a semiconductor wafer in a closed loop control system.Alignment devices of the type shown in U.S. patent application Ser. No.541,385 may be effectively used to provide feedback in the control loop.If it is important to minimize crosstalk between the X- and Y-axes, eachof flexure strips 65 should be equal in length and stiffness to flexurestrip 17. The point at which flexure strips 65 are attached to bar 67should be at the vertical midpoint of flexure strip 17, and PZT 35should be accurately aligned with the center of mask 1.

I claim:
 1. A flexure assembly apparatus for supporting a stage plateand X-ray lithography mask comprising:a pedestal; a horizontal arm;attachment means, connecting an end of the horizontal arm to a top ofthe pedestal, for allowing smooth rotation of the horizontal arm about arotational axis; displacement means, connected to an underside of thehorizontal arm, for supporting the horizontal arm and for selectablydisplacing the horizontal arm in a Z-axis; a rigid bar; vertical flexuremeans, connected between the rigid bar and another end of the horizontalarm, for allowing movement of the rigid bar in an X-axis; and horizontalflexure means, connected between the rigid bar and the stage plate, forallowing movement of the stage plate in a Y-axis.
 2. An apparatus as inclaim 1, wherein the attachment means comprises:first and secondvertical crossing flexure strips connected between the pedestal and thehorizontal arm; first and second horizontal crossing flexure stripsconnected between the pedestal and the horizontal arm; and wherein across point of the horizontal and vertical crossing flexure stripsdefines the rotational axis.
 3. An apparatus as in claim 2, wherein thedisplacement means comprises a piezoelectric transducer.
 4. An apparatusas in claim 3, wherein the vertical and horizontal flexure means eachcomprise a flexure strip.
 5. An apparatus as in claim 4, wherein eachflexure strip is fabricated from a spring material.
 6. An apparatus asin claim 5, wherein the spring material comprises hardened berylliumcopper.
 7. An alignment apparatus for movably supporting an X-raylithography mask, the apparatus comprising:a stage plate for holding themask in an X-Y plane; first displacement means, attached to the stageplate and disposed along a radial axis of the stage plate in an X-Yplane, the first displacement means being operative for linearlytranslating the stage plate along the radial axis; second and thirddisplacement means, attached to the stage plate and disposed in the X-Yplane orthogonally to the radial axis, for displacing the stage plate;and first, second and third flexure assemblies, attached to the stageplate at first, second and third points distributed equiangularly aboutthe perimeter of the stage plate, the flexure assemblies each beingoperative for supporting the stage plate and for displacing the stageplate along a Z-axis orthogonal to the X-Y plane.
 8. An apparatus as inclaim 7, wherein the displacement means each comprise an elastic elementattachjed to the stage plate, each elastic element being stiff in alongitudinal axis and flexible orthogonally to the longitudinal axis. 9.An apparatus as in claim 8, wherein the displacment means each comprisea piezoelectric transducer.
 10. An apparatus as in claim 9, wherein thefirst, second and third displacement means are anchored to first, secondand third fixed blocks.
 11. An apparatus as in claim 10, wherein thesecond and third displacement means are operative, in combination, fordisplacing the stage plate rotationally about a center and fordisplacing the stage plate linearly orthogonal to the radial axis. 12.An alignement apparatus for movably supporting an X-ray lithographymask, the apparatus comprising:a stage plate for holding the mask in anX-Y plane; first displacement means, attached to the stage plate anddisposed along a radial axis of the stage plate in an X-Y plane, thefirst displacement means being operative for linearly translating thestage plate along the radial axis; second and third displacement means,attached to the stage plate and disposed in the X-Y plane orthogonallyto the radial axis, for displacing the stage plate; first, second andthird flexure assemblies, attached to the stage plate at first, secondand third points distributed equiangularly about the perimeter of thestage plate, the flexure assemblies each being operative for supportingthe stage plate and for displacing the stage plate along a Z-axisorthogonal to the X-Y plane; said displacement means each including anelastic element attached to the stage plate, each elastic element beingstiff in a longitudinal axis and flexible orthogonally to thelongitudinal axis and a piezoelectric transducer; said first, second andthird displacement means each being anchored to first, second and thirdfixed blocks; said first flexure assembly including; a pedestal, ahorizontal arm, attachment means, connecting an end of the horizontalarm to a top of the pedestal, for allowing smooth rotation of thehorizontal arm about a rotational axis, vertical displacement means,connected to an underside of the horizontal arm, for supporting thehorizontal arm and for selectably displacing the horizontal arm in theZ-axis, a rigid bar, vertical flexure means, connected between the rigidbar and another end of the horizontal arm, for allowing movement of therigid bar in an X-axis.
 13. An apparatus as in claim 12, wherein theattachment means comprises:first and second vertical crossing flexurestrips connected between the pedestal and the horizontal arm; first andsecond horizontal crossing flexure strips connected between the pedestaland the horizontal arm; and wherein a cross point of the horizontal andvertical crossing flexure strips defines the rotational axis.
 14. Anapparatus as in claim 13, wherein the vertical displacement meanscomprises a piezoelectric transducer.
 15. An apparatus as in claim 14,wherein the vertical and horizontal flexure means each comprise aflexure strip.
 16. An apparatus as in claim 15, wherein each flexurestrip is fabricated from a spring material.
 17. An apparatus as in claim16, wherein the second and third displacement means are operative, incombination, for displacing the stage plate rotationally about a centerand for displacing the stage plate linearly orthogonal to the radialaxis.
 18. An apparatus as in claim 16, wherein the first, second andthird flexure assemblies are disposed along first, second and thirdradial axes of the stage plate.
 19. An apparatus as in claim 16,wherein: the vertical flexure means is substantially equal in length andstiffness to the horizontal flexure means; andthe vertical flexure meansis attached to the rigid bar at a vertical midpoint of the horizontalflexure means.
 20. An apparatus as in claim 19, wherein the second andthird flexure assemblies are substantially identical to the firstflexure assembly.
 21. An apparatus as in claim 16, wherein the springmaterial comprises hardened beryllium copper.